The rates of synthesis of the polyamines (spermidine and spermine) and their metabolic precursor putrescine are increased during the proliferative response of mammalian cells and elevated levels of these compounds are necessary for optimal cell growth. Two key enzymes of polyamine biosynthesis, ornithine decarboxylase (ODC) and S-adenosylmethionine decarboxylase (SDC), are regulated during this process. We have shown that the synthesis of these two proteins is regulated by both the levels and efficiency of translation of their respective mRNAs. In this application we propose to define the structure of the SDC gene and the mRNAs that are synthesized from it. The level of SDC mRNA is regulated post-transcriptionally and the mechanism of this will be studied. In addition, we will further characterize the mechanism of the control of translation of both ODC and SDC mRNAs through the use of in vitro mutagenesis and gene transfer techniques.